1. Field of the Invention
This invention relates to a semiconductor device with structure in which the semiconductor element thereof is connected to a circuit substrate via bump electrodes such as solder bumps.
2. Description of the Related Art
Conventionally, in order to assemble the semiconductor element with the circuit substrate, methods of connecting the tips of multiple outer leads of the semiconductor element (which is hereinafter referred to as a chip) to the metallization pattern on the circuit substrate, or mounting the semiconductor chip directly on the circuit substrate to connect them to the metallization pattern by wire bonding or TAB (Tape Automated Bonding), have been used.
However, the method of providing outer leads on the chip and mounting them on the circuit substrate a serious impediment to the development of high density assembly technology for semiconductor devices. Particularly, in recent years, various applications of semiconductor devices have been made and high density assembly technology is advancing. For example, since there is a trend for a thin circuit substrate to be used and a trend for the number of mounting memory chips to increase as in a memory card, the method of assembling the chip by use of the outer leads has a limitation. Therefore, a flip chip method of connecting multiple connecting electrodes (bumps) formed on the chip directly to the metallization pattern of the circuit substrate has received much attention.
In the flip chip assembly, a silicon chip has a pad electrode formed on the surface thereof and electrically connected to an inner integrated circuit and a plurality solder bumps with the height of approx. 100 .mu.m formed of low melting point metal on the pad. A plurality of chips are assembled on the circuit substrate by connecting the silicon chip to the metallization pattern formed on the surface of the circuit substrate with the chip surface set to face the circuit substrate. As a constituent material of the bump, gold may be used instead of the low melting point metal in some cases.
For flip chip connection using the low melting point metal, a method of preventing short circuits and collapse of the circuit element due to solder by making a portion of the metallization pattern other than a portion that is to be connected to the solder bumps non-wettable with respect to solder is disclosed in U.S. Pat. No. 3,401,126 Sept. 1968, L. F. Miller et al., and U.S. Pat. No. 3,429,040 Feb. 1969, L. F. Miller.
Generally, the temperature of a semiconductor device rises by heat generated from the chip at the time of operation. The heat generated from the chip is transmitted to the circuit substrate via the solder bumps to raise the temperature of the circuit substrate. At this time, the chip and circuit substrate thermally expand. In the flip chip connection, if there is a difference in the thermal expansion coefficient between the chip and the circuit substrate, thermal stress caused by the difference in the thermal expansion coefficient concentrates on the solder bump.
If no encapsulant is filled in between the chip and the circuit substrate, or an encapsulant having not so large Young's modulus is uniformly filled, the relative positions of the solder bump on the chip and the circuit substrate vary according to a difference between the thermal displacements thereof. The reliability of the solder bump depends on the reciprocal of a difference in the displacement between the upper surface and the lower surface of the solder bump caused when the semiconductor device is repeatedly operated. That is, as the difference in the displacement is larger, the stress more strongly concentrates on the solder bump and the long-term reliability of the semiconductor device is more degraded. Since the difference in the displacement varies in proportion to the distance from the center of the chip to the solder bump, it becomes more difficult to attain high long-term reliability in the present condition in which the integration density tends to be enhanced and the chip size tends to be increased.
Conventionally, the encapsulant filling method for stress alleviation is effected by temporarily fixing the chip on the metallization pattern of the circuit substrate, causing the solder bump to reflow so as to connect the same to the metallization pattern, then flowing epoxy or polyimide encapsulant into between the chip and the circuit substrate and curing the same. In this case, the result of evaluation of the reliability for epoxy is described in "Microelectronics Packaging Handbook", page 389, Van Nostrand Reinhold, 1989, R. R. Tummala et al.
Further, since the Young's modulus of silicone is too small, it is not used as the stress alleviating encapsulant in the prior art. In the encapsulated flip chip connection, in order to alleviate the stress concentrating on the solder bump, optimum material properties must be selected as the material properties of the encapsulant. The material properties of the encapsulant depend on the amount of fillers contained therein. The size of the filler is several .mu.m to several ten .mu.m and is normally 10 .mu.m to 20 .mu.m.
However, if the pitch of the flip chip connection is reduced with the trend of high integration density, the space between the solder bumps and the space between the chip and the circuit substrate are also made smaller. At this time, even if an encapsulant whose material properties such as the Young's modulus are optimized is selected, there occurs a problem that the encapsulant cannot enter a space between the circuit substrate and the chip since the size of the filler contained in the encapsulant is large. This problem becomes particularly important when the connection pitch is reduced to 100 .mu.m or less.
Further, in the conventional flip chip assembly method, the chip and the circuit substrate are fixed mainly by the adhesive force of flux in a period from the time when the chip is temporarily fixed on the circuit substrate until the chip is connected by reflowing. At this time, as a material property of flux, adhesive force is necessary. A flux having strong adhesive force exhibits narrow spreading when it is coated; and the manufacturing efficiency thereof is low. Further, when the connection pitch is small, it is difficult to stably coat the flux with a small thickness on all of the electrodes. Further, in the fluxless connection, the fixing force of the chip on the circuit substrate is attained only by adhesive force of the solder obtained when the solder bump is pressed on the circuit substrate; and it tends to be unreliable.
As the other flip chip connection method, a connection method by gold-gold pressure is used. Also, this method includes a step of temporarily fixing the chip by coating an adhesive agent before alignment of the chip and the circuit substrate, but in the flip chip connection using the solder bump, the self-alignment effect can be expected when the solder bump is caused to reflow, and therefore, it is not preferable to fix the chip by use of adhesive agent before the reflowing as in the case of the connection method by gold-gold pressure.